Jun Zha

Effect of working position on vertical motion straightness of open hydrostatic guideways in grinding machine

Hydrostatic guideways have various applications in precision machine tools due to their high motion accuracy. The analysis of motion straightness in hydrostatic guideways is generally ignoring the external load on the slider. A variation force also exists, caused by the different working positions, together with the dead load of the slider and that of other auxiliary devices. The effect of working position on vertical motion straightness is investigated based on the equivalent static model, considering the error averaging effort of pressured oil film in open hydrostatic guideways. Open hydrostatic guideways in LGF1000 are analyzed with this approach. The theoretical results show that the slider has maximum vertical motion straightness when the working position is closer the guiderail of Y axis. The vertical motion straightness reaches a minimum value as the working position is located at the center of the two guiderails on the Y axis. The difference between the maximum and minimum vertical motion straightness is 34.7%. The smaller vertical motion straightness is attributed to the smaller spacing of the two pads centers, along the Y direction. The experimental results show that the vertical motion straightness is 4.15 μm/1200 mm, when the working position is located in the middle of the X beam, and 5.08 μm/1200 mm, when the working position is approaching the Y guiderails, denoting an increase of 18.3%. The changing trends of the measured results validate the correctness of the theoretical model. The research work can be used to reveal the variation law of accuracy of the open hydrostatic guideways, under different working positions, to predict the machining precision, and provides the basis for an error compensation strategy for gantry type grinding machines.

Precision design of hydrostatic thrust bearing in rotary table and spindle

According to the increasing needs of rotary table and spindle to satisfy high-precision machining requirements, the accuracy of rotary table and spindle becomes an important issue due to the error averaging effect of hydrostatic thrust bearing. The objective of this study is to research a methodology to guide the precision design of hydrostatic thrust bearing in rotary table and spindle. A run-out error model based on error averaging effect is established using the Reynolds equation, pressure boundary conditions, flux continuity equations of pad and dynamic equations of shaft. The axial run-out error and angular error are calculated considering perpendicularity error and flatness error of the components. The simulation results show that the two perpendicularity errors between axis line and thrust bearing bushing surface have same direction, and the axial run-out error could reach to the maximum values. Also, the flatness error of thrust bearing bushing surface has a big influence on axial run-out error. Following the outcomes, the precision design of hydrostatic thrust bearing was conducted. The axial run-out errors of rotary table and spindle with hydrostatic thrust bearing were experimentally studied, and the results have good coherence to the simulation data. The run-out error model is demonstrated to be an effective approach to guide the precision design of hydrostatic thrust bearing in other rotary tables and spindles.

Relationship between elliptical form error and rotation accuracy of hydrostatic journal bearing

Purpose The form error of shaft and hole parts is inevitable because of the machining error caused by rotation error of tool axis in machine tools where the elliptical form error is the most common in shaft and bearing bush. The purpose of this paper is to present the relationship between the elliptical form error and rotation accuracy for hydrostatic journal bearing in precision spindle and rotation table. Design/methodology/approach An error averaging effect model of hydrostatic journal bearing is established by using Reynolds equation, pressure boundary conditions, flux continuity equation of the land and kinetic equation of shaft in hydrostatic journal bearing. The effects of shaft and bearing bush on rotation accuracy were analyzed quantitatively. Findings The results reveal that the effect of shaft elliptical form error on rotation accuracy was six times larger than bearing bush. Therefore, to improve the rotation accuracy of hydrostatic journal bearing in spindle or rotation table, the machining error of shaft should be controlled carefully. Originality/value An error averaging model is proposed to evaluate the effect of an elliptical form error on rotation accuracy of hydrostatic journal bearings, which solves the Reynolds equation, the flux continuity equation and the kinetic equation. The determination of form error parameters of shaft and bearing bush can be yielded from finding results of this study for precision design of hydrostatic journal bearings.

Uneven Preload for Improving Motion Straightness in Closed Hydrostatic Guideways

Hydrostatic guideways have varied applications in precision or ultra-precision machine tools due to their high motion accuracy and low friction coefficient. Slider motion straightness is an important accuracy index for evaluating precision of hydrostatic guideways. It can be defined as the linear deviation of the slider center within the entire or any assigned stroke. This paper mainly focuses on the relationship between the keeper rail (upper guiderail) preload and slider motion straightness in the closed hydrostatic guideways, where the slider is enveloped by the guiderails. A finite element method was used to calculate keeper rail deformation under different screw preloads, slider motion straightness was measured by laser interferometer. Mapping relationship between screw preload and slider motion straightness was obtained. Experimental results shown that the end of the keeper rail had larger deformation for same preload conditions on all screws. This had an obvious effect on slider motion straightness. When oil supply pressure was 0.85 Mpa, slider motion straightness was 2.08 μm and 1.85 μm (within the entire stroke 330 mm) with torque on all screws as 15Nm and 20Nm, respectively. After uneven preload was employed, slider motion straightness was 0.87 μm and 0.84 μm for the above mentioned two cases. Accuracy increased by more than 50 percent. Screw preload approach proposed in this study can be used to guide the assembly processes of closed hydrostatic guideways with other forms for improving the motion straightness.Copyright

Precision Prediction Model and Experimental Verification of Hydrostatic Bearing-Rotor System of Ultra-Precision Machine Tools

Error averaging effect of the pressured oil film allows for a high motion precision in hydrostatic bearings. This has many applications to ultra-precision machine tools rotor-bearing system. This paper mainly focuses on the studying factors affecting the rotor-bearing system error for hydrostatic rotation table and hydrostatic spindle in ultra-precision grinding machine. Radial run-out error was selected as precision evaluation index. A precision prediction model was established, which takes into structural and dynamic parameters of hydrostatic radial bearing rotor-bearing system into consideration. Effect of assembly tolerance, structural parameters and working conditions on the accuracy of the rotor-bearing system were analyzed. Precision measurements for the ultra-precision hydrostatic rotary table and spindle were carried out. Prediction errors of radial run-out error was found to be less than 23.5%. The model proposed in this paper has high prediction precision and can be used to guide the precision design and optimization of the ultra-precision machine tools rotor-bearing system.Copyright